The present invention relates to a supervisory system for a primary group digital transmission line system, such as a PCM-30 channel or PCM-24 channel system. FIG. 1 illustrates a structure of an ordinary digital transmission line (inter office junction line) of a PCM-30 channel system. Office line signals are output on outgoing trunks of an exchange. Up to 30 of these signals (channels) are multiplexed, by a multiplexer apparatus (MUX), and are provided as, for example, a 2 Mbps digital signal. The multiplexed digital signal is amplified by a terminal repeater LTE0 and is then transmitted to an up-inter office junction line comprising, for example, a 2-wire transmission line.
In general, the inter office junction line includes intermediate repeaters REP0, 1, . . . n which are typically spaced 2 km apart. Therefore, a digital signal is amplified to a specified level by each intermediate repeater (REP0, 1 . . . n), and is then transmitted to a terminal repeater LTE1. In the case of transmitting a signal to the terminal repeater LTE0 from the terminal repeater LTE1, a signal is transmitted through a down-inter office junction line comprising a 2-wire signal line, and is then separated into each channel by a demultiplexing apparatus (DMUX). The demultiplexed signal is then applied to the exchange through incoming trunk lines. Terminal repeaters such as LTE0 and LTE1 are provided at, for example, a point where repeater line are joined, at an inter office junction line branch point or at the input/output of office lines from an exchange as in the case of LTE0.
Since the digital transmission system illustrated in FIG. 1 must operate with high reliability, it is necessary to monitor the system for faults generated in the terminal repeaters LTE0, LTE1, intermediate repeaters REP0, 1 . . . n, and the inter office junction lines. It is also necessary to detect the fault location when a fault occurs. Therefore, monitoring units MON0, 1 . . . n, are respectively provided for the intermediate repeaters REP0, 1 . . . n. The supervisory units SV0, SV1 are respectively provided for the terminal repeaters LTE0, LTE1. The supervisory units and the monitor units are connected by a supervisory line which is used for the monitoring process.
Such monitoring must be carried out effectively so as to detect fault locations as quickly as possible. In existing primary group PCM transmission lines, fault location of a line is performed by a pulse trio allowance test which can be performed only when the line is out-of-service.
FIG. 2 is a block diagram of a structure of an existing supervisory system, and FIG. 3 illustrates output signals of the pulse trio generator used in the existing system. Referring to FIG. 2, in order to locate a fault in the terminal repeater 13, the intermediate repeaters 14-1, 14-2, 14-3 . . . n (where n equals, e.g., 24 repeaters) or in the inter office junction line, the inter office junction line must first be set to an off condition; that is, placed in a non-operating condition.
To monitor the status of the terminal repeater 13, an output of the pulse trio generator 12 is applied to an input terminal 19-1 of a transmitting circuit (hereinafter "T") of the terminal repeater 13. The pulse trio generator 12 changes a DC level (FIG. 3(d)) to a constant frequency signal (FIG. 3(c)) comprising pulse trio signals "1," "0," "-1," "0," "1" (FIG. 3(a)) "-1," "0," "1," "0," and "-1" (FIG. 3(b)). There are 24 typical patterns in the voice frequency band that can be obtained by changing the repetition pattern of the pulse trio signal indicated in FIGS. 3(a) and (b) (e.g., 1005 Hz-3016 Hz). For instance, a pulse trio signal, having a DC level variation frequency f.sub.1, (for example, 3016 Hz) is applied to the input terminal 19-1 of the terminal repeater 13. This signal passes through T.sub.1 and is applied to a band-pass-filter (BPF) 15, which allows signals having the frequency f.sub.1, and neighboring frequencies to pass. The output of BPF 15 is applied to a selection level meter 21. The selection level meter 21 measures the level of the received signal having the frequency f.sub.1. This level is then compared to the applied signal level output from the pulse trio generator 12. When the applied signal level is equal to the received signal level measured by the selection level meter, there is no fault in T.sub.1. On the contrary, if the signal level measured by the selection level meter is different from that of the applied signal level by more than a specified amount, there is a fault in T.sub.1. Fault location detection can also be applied in the same manner to T.sub.2 of the terminal repeater 13 by applying an output of the pulse trio generator to the input generator 19-2.
Next, such fault location is conducted for the intermediate repeaters 14-1, 14-2 . . . 14-n. First, a pulse trio signal having a DC level variation frequency f.sub.2 (e.g., 2792 Hz) is applied to the input terminal 19-1. The band-pass-filter BPF 15 of the terminal repeater 13 does not pass the frequency f.sub.2. Band-pass-filter BPF 16, however, does pass the frequency f.sub.2. The signal therefore passes through T.sub.1 of the intermediate repeater 14-1 and then to the selection meter 21.
In the selection meter 21, the applied signal level output from the pulse trio generator 12 is compared to the received signal level measured by the selection level meter. Fault location in T.sub.1 of intermediate repeater 14-1 is conducted as explained above. Such fault location is also conducted for T.sub.2 and other repeaters up to, for example, 36 units. A maximum of 24 repeaters (terminal and intermediate) can be included in the system when an intrinsic frequency is assigned to each repeater. This is because there can be a total of 24 different frequencies generated by the pulse trio generator 12.
Fault location detection for a receiving circuit (hereinafter "R") can also be realized by applying the signals having frequencies f.sub.1, f.sub.2 . . . f.sub.24, as explained above to the proper input terminals of each repeater.
The inter office junction line is usually very long and includes many repeaters. The repeaters are needed because the repeating distance for a 2 MHz PCM transmission line is approximately 2 km. The test of successive repeaters is conducted by changing the frequency of the applied signal output from the pulse trio generator 12, which is used as a supervisory signal. As noted above, the frequency of the applied signal (i.e., the supervisory signal) can be any one of 24 frequencies within the voice frequency band of approximately 1 kHz-3 kHz. Thus, each repeater is assigned one of the 24 frequencies, and the repeaters are individually tested from, for example, the terminal side. Therefore, the FIG. 2 system can test no more than 24 repeaters.
In addition to the above limitation, the supervisory line L does not include any amplifiers and therefore attenuates the pulse trio signal provided by the pulse trio generator 12. If supervisory equipment is provided at each end of the transmission line, then a supervisory line L from each end need only be 1/2 the full distance between the offices. The problem of signal attenuation limits the maximum distance between offices. Also, the above test is carried out by applying the pulse trio signal to the transmission line system. As a result, the test cannot be conducted while the system is in service. Monitoring during normal, in-service conditions is impossible, and therefore, service must be suspended in order to test the system.